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Original Message

Re: Old design detail on PPP tubes.......

Posted by Tom Bavis on February 20, 2005 at 13:00:15:

They are there to supress parasitic oscillations in the VHF/UHF range. The tubes have plenty of gain there, and the connecting wires have a resonant frequency in that range... so a parallel audio circuit becomes a push-pull UHF oscillator! Sometime a low value resistor is enough, sometimes it's a combination, often a choke wound on a 47 Ohm resistor.

Not all amps use them... not all amps are stable, either...